Process of selective crystallization



W. E. BURKE ET AL PROCESS 6F SELECTIVE CRYSTALLIZATION Filed March a,- 1930 Calf wound Alp/1 a V W 7m oera fare Patented Dec. 8, 1 931 1 ame STATES Parser i rics WILLIAM BUEKE, CHARLES E.

RITCHIE, AND ROBERT 13. FEET,

rennin, ASSIGNORS fro AMERICANPOTASH & CHEMICAL conrona'rroir, 0F 'rnoim,

cALrEoEn A, A CORPORATION or DELAWARE PROCESS OF SELECTIVE CRYSTALLIZATION Application filed March 3,

This invention relates to a new and useful process of selective crystallization, by which individual compounds may be separated from a solution containing two or more crystallizable compounds, In many instances there are encountered solutions con1 tainingtwo or more soluble compounds, the solubility characteristics of which are simi lar. One of the most common methods for recovering soluble compounds from solution by cooling. It a solution containing two or more compounds is cooled below the point "of saturation with respectto compounds hav- 12 similar solubility characteristics, a precipitate comp ising the two compounds will result. Since puresubstances are generally desired in commerce, this feature is very undesirable. Tedious, ineflicient and costly means of fractional separation have been resorted to in many such cases in the past, in order to prepare pure compounds from such systems.

. It is an ect of the present invention to provide means by which compounds having iimilar solubility characteristics may be crystallizedirom solution and separatedone from the other in a simple, economical and effective manner. I I

[i a specific example, referring to Figure Lassume asolution to containC parts of compound alpha and B parts of compound beta, the solution existing at a high temperature, X said temperature'being suflicientto maintain both compounds, alpha and beta,in' the solvent. The solution may be cooled to Y Without precipitating either compound. Bvrfihe usual means of fractional crystallization, a small quantity of compound alpha may be crystallized from mlution in pure form by. cooling from Y- to V By ordinary method,-'known and practiced in the past, further cooling from YJ to Z for the purpose of further crystallizing fromthe solution additional quantities of compound alpha, results in the pre- 1930. Serial No. 432,904.

cipitationof appreciable quantities of com-.

pound beta. 7

By the process of this invention, sible to cool the solution from its elevated temperature, X", to the desired low temperature, Z and to crystallize therefrom an essentially pure crop of compound beta and an essentially pure crop of compound alpha, said pure crops being amenable to saturation have been set forth. A Well known concept is the definition of the metastable field (and metastable limit) of supersaturation'as opposed to the labile field ofsupersaturation. Generally, the term metastable field of supersaturation is employed to de-:

fine the condition of a supersaturated system' in which the degree of supersaturatlon of the solute is insufiicient to bring about spontaneous formation of crystal nuclei. The labile field is said to be that regionbeyond the metastable limit of supersaturation in which'the' degree of supersaturation of the solute is sufficient to cause the spontaneous formation of crystal nuclei.

While in the past certain'very definite lines of demarcation have been employed in applying such concepts to various systems, We

are of the belief that such phenomena com prise continuous functions, the constants of which,'of course, vary with different systems. We believe that all solutes are capable of somedegree of supersaturation in all solvents, that is to say, any saturated system may, under proper conditions, be supercooled to some degree without immediately precipitating the or TRONA, CALI- itis posfor the appearance of crystal nuclei,

solute. We admit, however, that the degree of supersaturation which it is permissible to incur without exceeding the so-called metastable limit of supersaturation varies over an extremely wide range, and that in many instances said degee is so slight as to be practically negligible. However, we have found that such solutes as potassium chloride and sodium chloride, previously considered as incapable of undergoing supersaturation in aqueous solution, are capable of measurable, though small, degrees of supersaturation under proper conditions. Solutes of the class comprising hydrated sa ts, doub e salts, and certain organic compounds are, on the 0th r hand, capable of quite a great degree of metastable supersaturation. 7

Pursuant with our theory of the continuity of supersaturation phenomena, 'we believe that the so-called metastable lin it of supersaturation constitutes only a special or arbitrary demarcation of a continuous function. o have found that the factors influencing the stability of a supersaturated system, that is, those which define the so-called metastable limit, include the specific nature of the solute and the SOlVQni time, and the nature of the equipment employed. The concept of the time required for the appearance of crystal nuclei after incuring supersaturation in a system, is one of considerable importance and strongly verifies our theory of continuous functions as opposed to the former prece ts embracing the discontinuous idea. For example, if a system such as boric acid in water be supersaturated by coolin in a sealed glass tube under sterile conditions, the time required for the formation of crystal nuclei found to be a function of the degree of'supersaturation incurred; the higher the degree of supersaturation, the more rapid the appearance of crystal nuclei. Usually, a plot or graph of such d wherein degree of supersaturation is plotted against time required results in a curve of the general form of a rectangular hyperbole. Since such a curve becomes asymtotic to both axes it may be seen that in the case of high degrees of supersaturation the time required for nuclei formation is negligible, i. e. the system is labile, while in the case of low degrees of supersaturatirm the time required for nuclei formation becomes infinitely long, and such a system said to be metastable. The metastable limit of supersaturation may be taken at some point where the time required for nuclei formation becomes relatively short or shorter than is allowable for the fulfillment of certain independently fir-zed conditions. It is to this definition of the metastable limit of su persaturation that we refer in the following exposition.

The concept of equlpment affectlng the metastable limit of supersaturation is likewise of importance. It has been found that the time required for the induction of crystal nuclei varies with the conditions employed: a. system may be found to be metastable under one set of conditions and labile under another. supersaturated solutions are extremely sensitive to mechanical stimulus. Mechanical stimulus may be brought about in a large number of manners, such as, for example, the setting up of strong impulse waves within the system, and by the force of two substances rubbing the one against the other. A well known example of the latter is the trick employed by the chemist, of scratching the inside of a beaker with a stirring rod to induce crystallization from supersaturated solution. In general, the more supersaturated the system, the more labile it becomes under conditions of mechanical stimulus. While it has long been considered that in order to produce metastable systems, freedom from crystal nuclei is essential, this is not necessarily true in total. We hav been able to )roduce supersaturated systems in the presence of crystals of the supersaturant,which, (with the exception of the normal deposition upon the crystals present), wereentircly metastable and could be so maintained over long periods of time. However, we have found that mechanical stimulus, under such conditions is most active in destroyin supersaturation or, in other words, in decreasing the metastable limit of supersaturation. This phenomenon, like others described, comprises a continuous function; that is, it is dependent upon the degree of mechanical stimulus involved.

t is upon the foregoing principles that the present invention is predicated. In short our invention comprises crystallizing in a common vessel and in the presence of each other the two compounds alpha and beta, having similar solubility characteristics. By the process of this invention, one compound is caused to form upon a predetermined seed bed in the form of large crystals, while the other is caused to form as relatively fine crystals, which are subsequently separated from the large crystals by suitable means, such as hydraulic classification, sreeningr, sedimentation, etc. While the steps of the process may be carried out in a batch or in a continuous manner, the latter is preferable from the standpoint of economy, labor and control. Hence, the followingz exposition is set forth in such terms.

By the operation of this invention, the solution described above with reference to Figure 1 is cooled to Z", thereby producing supersaturation (B-D parts) with respect to compound beta. The supercooled solution is then passed into a suitable crystallizer, the details of which are described hereinbelow, where n the supersaturation value of compound beta (B-D parts) is caused to deposit upon a seed bed of already formed beta crystals, while the content of compound alpha is carried from said chystallizer as a suspension of line crystals.

-tion should be such that the effect of mechanical stimulus brought into play in the crystallizer is small as compared with the rate of crystal growth of the supersaturant upon the seed bed of compound beta in the crystallizer.

By taking advantage of the opposite'phenomenon, compound alpha is caused to pre cipitate as fine crystals, being, in the case of continuous operation, subsequently removed from the crystallizer by the flow of liquor. ln cooling the solution to Z", it is eccssary that a state of supersaturation with respect to compound alpha be produced which is labile. By labile, we here refer to a system supersaturated to such an extent that the time element required for nuclei formation, either spontaneously or abetted by mechanical stimulus and the presence of like crystals, is so small as to cause the desired results, as herein described, to be brought forth.

The production of a labile state, of course, depends to a large extent upon thesolute itself. In many cases the concentration. of compound alpha, C parts, is suflicient to produce a labile system with respect to that compound when the solution is cooled to Z". However, we have encountered cases in which compound alpha-possessed supersaturation characteristics similar to those of compound beta. In such a case, it may be advisable to add to the solution, prior to the described manipulations, a suflicient quantity of one compound, alpha for example, to produce the desired degree of supersaturation, that is, to produce a labile system with respect to compound alpha' For example, A-C parts of compound alpha may be added to a unit quantity of the solution, said solution being at X to produce a system containing A parts of compound alpha and B parts of compound beta. This solution, wnen cooled to Z", acquires a relatively great degree of supersaturation (ll-E parts) with respect to compound alpha and a relatively small degree of super saturation (BD parts) with respect to compound beta, and behaves in the manner 5 desired when processed asjust described.

The action just described is brought about 'With the system so-adjusted as to possess a relatively stableor a metastable degree of supersaturation with respect to compound beta and an instable or labile state with respect to compound alpha, the cold solution is caused to pass through a bed of relatively large crystals of compound beta. This is best accomplished by introducing the liquor atthe lower extremity of a cylindrical crystallizing vessel in such a manner that the liquor is caused to flow upward through the crystal bed of compound beta. This crystal bed serves as a medium upon which the metastable beta content of the liquorniay be deposited in an orderly and controlled fashion. C-n the other hand there are no large seedsor crystal bed of compound alpha. provided within the crystallizer. Hence, in the usual manner of labile solutions, spontaneous crystallization talres place, with an attendant formation of an enormous quantity of extremely fine crystals. This formation of line crystals is promoted by the mechanical stimulus set up within the 'crystallizcr by suitable agitators together with the internal abrasion set up by the rapid passage of the liquor between the interstices of the suspended seed crystals of compound beta.

these means the crystallizable content ofcompounc. alpha is caused to precipitate as fine crystals. These fines are caused to be carried upward with the liquor stream, and therewith carried from the crystallizer. The line crystals of compound alpha may be subsequently recovered from the mother liquor by sedimentation, filtra ion, centrifugation or other known and suitable means. In this manner the process of the present invention serves to separate from av solution, simultaneously separating the one from th other, two substances having similar solubility charac teristics." V i The type of'crystallizer employed is of little importance as long as the conditions imposed by the system involved are successfully fulfilled. The volume of the seed bed, of

beta crystals for instance, must be sufficient to insure satisfactory removal of the supersaturation of compound beta from the liquor. The ratio of height of the crystallizer to its volume is determined by the size of the crystale of compound alpha produced. It is requisite of the present system that the crystals of compound alpha produced'inthe cooling and crystallizing operations shall be substantially smaller'that those of compound beta. in the case described, these are removed from the crystallizer and from the beta crystals by means of the liquor stream. Hence, the size the alpha crystals produced determines the minimum velocity of upward low of liquor required for their hydraulic removal. If the alpha crystals are fine, a relatively slow movin g stream of liquor is sufficient; if relatively demanded. These functions of course are in part dependent upon the solute involved and its degree of supersaturation or concentration, as well as the density and viscosity of the mother liquor.

The choice of the method of cooling is like wise a consideration dependent upon specific conditions. The conventional type of double pipe cooler has been found quite satisfactory in the case of continuous operation. This type of cooler is shown somewhat diagrammatically in Figure 2. Hot liquor from a suit-able storage tank 19 is passed by a pump 18, into the inner pipes of a double pipe coolor unit 20. Cold brine or other suitable cooling medium is introduced into the outer pipes at point 16. Cold brine passes counter-currently to the hot liquor and leaves the unit at point 15.

Single pipe coolers immersed in or splashed with cooling medium have also been em ployed. Preferably the cooling units siiould be'external to the crystallizer proper, in order to facilitate cleaning and repairing; but, in certain instances, the cooling means may be incorporated within the crystalliaer itself. In other cases coolin of hot liquor b Y subjecting the same to reduced pressure has been found very economical and advantageous.

According to the precepts of supersaturation hercinbefore set forth, the rate of discharge or precipitation of the supersaturw tion with. respect to compound alpha depends upon a number of factors. In many instances the precipitation is almost instantaneous, even to the extent of taking place within the cooling units. In other cases where compound alpi a shows a strong tendency towarc metastable supersaturation, th rate of precipitation is less rapid. It is paramount that a'large quantity of crystal nuclei or seeds of compound alpha be present within the crys tallizer, which seeds may be f0 med by means of rapid and spontaneous crystallization. As 'he create-1' portion of the supersaturation is discharged in this form, the system may resolve itself into one con" ining small supersaturation values, both with respect to com pound alpha. and compound beta. Such conditions are proper for the orderly growth of these substances upon their respective like seeds, without further spontaneous crystallization. However, at this point, there must be present within the crystallizer a very large number of very line cr stal nuclei of compound alpha, upon whicn the residual small degree of supersatruration may be deposited. 'lhis relatively small addi-..ion of ma to th fine crystal nuclei of compound alpha already present is so small as compared to the total mass of the same, that the size of the individual particles is not materially increased thereby.

In certain cases where it ln s been impera- 1 tive to cool the solution to Z for the removal of stated quantities of the compounds, it has been found that the degree of supersaturation with respect to compound beta so produoed (BD parts) was in excess of the metastable That is to say, with such a de 'ree of supersaturation orderly deposition of compound beta upon the seeds thereof in the crystallizing vessel would not take place; but spontaneous crystallization ensued, causing fine crystals of compound beta to be formed and carried from the crystallizer with the outgoing liquor together with the finc crystals of compound alpha. Since the operation of this invention depends upon the principles of forming large crystals of one compound upon a prec etermined seed bed and the crystallizing of the other compound in the form of fine crystals, amenable to physical separation from the former; the aforementioned result would tend to defeat the purpose of this invention.

To circumvent this fault we first cool the solution con aiming, for example, A parts of compound alpha and B parts of compound beta from its elevated temperature X to some temperature V", short of Z". By cooling only to V instead of Z a lesser degree of supersaturation with respect to compound bet produc d. said degree of supersaturan being su icient to prevent spontaneous L1. crystal isation or nuclei formation of compound h insuring an orderly growth of the sar 1e upoi the seed bed within the crystallizer. This solution is passed to the crystallizer where the deposition of compound beta proceeds, the desired crystallization of fine crystals of compound alpha also taking place. Solution from which the supersaturation values with respect to V of compound beta have been removed, may then be further cooled to a lower temperature (e. g. to Z") and passed to another crystallizer for a re, etiti-on of the process.

In certain cases it may be necessary to reheat the outflowing liquor from the first crystallizer from V to X and subsequently recool the same to some temperature below V (e. to Z") prior to passing the same to a second crystallizcr. However, in most cases it is only necessary to cool the liquor leaving .he first crystallizer at V" down to Z". This liquor, of course, carries in suspension the entire crop (AB parts) of compound alpha precipitated at V in the form of very fine crystals. If these crystals are of such number or size that the further addition of BE parts compound alpha thereupon does not materially alter their mass; then further heating to X and recooling to Z becomes unnecessary. This will evidently be true even when the degree of supersaturation of BE iarts of compound alpha, produced by cooling; from V to Z, is within the metastable field of supersaturation.

An examination of the number-size-mass relationships of, crystals is enlightening. If

seeds of even relatively large diameter, e. g. 100 +150 mesh, having an average diameter of 0.12 millimeters, are formed in the irst stage of cooling from X to V", by the 1 stallization of AB parts of compound alpha. then it would be necessary to add thereto approximately three times the weight of material already brought out of solution,

to g1 u these seeds from -100 +150 mesh to 65 +100 mesh, which corresponds to an average diameter of about 0.18 millimeters. Thus it is seen that approximately 300% increase in mass is required to effect a 50% incr ase in diameter of the individual crystals.

Considerations of the above nature have led us to another modification of the process of our invention, which has proven very useful in certain cases. By means of this modification, we do not have to depend entirely upon thespontaneous formation of a multiplicity of fine seeds from the major body of the liquor. For example, referring to our original example of a liquor containing C parts of compound alpha and B parts of compound beta, this liquor is cooled to Y or lW6l,'lI1Cl there is added thereto a suflicient weight, say, for example, A; of C-E parts of fine (-100 +150 mesh) crystals of compound alpha. A very much lesser quantity of finer alpha crystals may be added if such are more convenient. The resulting treated solution. may then be cooled and crystallized as described hereinbefore, causing the precipitation ofthe metastable compound l 1 upon largeseed crystals of the same contained within the body of the crystallizer, further causing the precipitation of the content of compound alpha as fine crystals which are subsequently separated from the large crystals of compound beta (in the case of continuous operation, by means of the upflowing stream of liquor).

The fine crystals of compound alpha, added in such number that the total growth of alpha. from the solution thereupon will not materially the individual size thereof,

may be produced in a variety of ways. A

s "nple and obvious method comprises grindlar e crystals .of compound alpha, either .i a suitable mill as wet plup. A v satisfactory method for producing the red quantity of fine alpha crystals is ad upon the principles set forth hereinbef. A small quantity of asuitable solrent is saturated with alarge amount of compound a pha at a suitably high temperature, say X or higher. This hot concentrated liquor is then cooled rapidly to some suitable temperature, for instance. Mechanical stimulus, high supersaturation and rapid cooling are employed to bring about suontaneous crystallization of a multiplicity 8 (Es-B) parts compound alpha, in order of the desired crop of fine alpha crystals spontaneously, solely by the production of a highly labile system. By the preferred means only a relatively small quantity of material must be handled, heated and cooled, as compared with treating the entire body as aforedescribcd. The former process, however, possesses practical utility in cases where recrystallization of impure compound alpha must be realized, and may be accomplished to advantage by combination with such a step.

In the practice of this invention it is necessary to. supply to the crystallizer relatively large beta seed crystals. Except in rare cases, the beta seed crystals are not formed spontaneously and furthermore the volume and size thereof must increase with the eX- traction of beta values from the metastable solution. Therefore, it is usually necessary to continually add to the crystallizer, seed crystals of compound beta and to continually remove therefrom the enlarged crystals.

In the case of continuous operation, the size of the seed crystals of compound beta is dependent upon-the rate of flow of the liquor. These seed crystals must be of suflicient size to remain within the crystallizer against the force of the upward flowing stream ofliquor. lVe prefer to utilize the smallest beta seed possible in order to take advantage of the greatest growth factor, and consequently the least addition of seeds. The linear velocity of the upward stream of liquor, for a given volume flow, depends upon the size of the alpha crystals present; hence, the minimum of the beta seed crystals is a dependent, rather thanan independent, function of the system as a Whole. 'We have found crystals of size 28 mesh or about 0.71 millimeters diameter to be suitable as seed in most cases. lf'these crystals are grown to size 8 10 mesh or about 2.0 millimeters diameter (or side length), it is necessary to supply approximately 100 pounds of the finermesh crystals (0.71 mm.) .forevery ton of large (2.0 mm.) crystals removed from the crystal- 'lizer.

to be understood that such details as herein set forth are not to be construed as limitations of the scope of this invention, that the invention rests solely upon the principles herein set forth and as claimed hereinbelow.

As an example of the application of this invention to one specific problem of selective crystallization, we will describe it as we have applied it, in one instance, to the separation and recovery of borax,

nanpaion o,

and the hydrated double salt of trisodium phosphate and sodium metaborate,

Na PO .NaBO .18H O,

from liquor obtained by the concentration and manipulation of Searles Lake brine. This concentrated liquor contained 250 grams of boraX (10.4%Na B O and 55 grams of the double salt (O.5 l%P O per liter, together with considerable quantities of sodium chloride, sodium carbonate, sodium metaborate, potassium chloride, organic matter and other impurities and had an approximate specific gravity at 60 C. of 1.3. This liquor was cooled from approximately 60 C. to a point sufliciently below 225 C., that the liquor emerging from the crystallizer at point 13, Figure 2 was approximately 225 C. Several degrees of excess cooling must be supplied to counteract the efiect of the heat of crystallization of the salts in question. The cold liquor was passed through a crystallizer of the general construction and arrangement illustrated in Figure 2 of the drawings containing a bed of crystals let, or the hydrated double salt of trisodium phosphate and sodium metaborate. The crystallizer had a horizontal cross section of two square meters. The vertical distance between the centers of the liquor inlet and the liquor outlet, (distance from 17 to 13, Figure 2), was 12 meters. The bed of crystals of the hydrated double salt of trisodium phosphate and sodium metaborate was maintained so that when the cooled liquor was supplied to the bottom of the crystallizer at the rate of approximately 200 liters per minute the upper surface of the seed bed stood about 2 meters below the liquor outlet 13, Figure 2. At the rate of 200 liters per minute, the average upward linear velocity in the upper two meters of the crystallizer (above the seed bed) was centimeters per minute. Throughout the seed bed the linear rate of flow was considerably greater than 10 centimeters per minute, being dependent upon the size of the crystals in the bed. The crystals of the double salt in the seed bed were maintained so that a screen analysis showed them to lie between the range 01" 8 mesh and mesh, that is to say, the crystals all passed through the opening of the 8 mesh screen, 2.36 millimeters, and none of them passed through the opening of the 35 mesh screen, Ail? millimeters. Crystals of such size as would pass through a mesh screen opening, 0.10 I millimeters, were found to be carried out of the crystallizer by the upward flow of 10 centimers per minute of liquor leaving the crystallizer, the boraX crystallized being so removed.

At the net effective ten'iperature of cooling, 225 C. the liquor contained an equivalent supersaturation of about grams of boraa and about 35 grams of the double salt per liter in excess of the amount of these substances in approximate equilibrium with the solids at that temperature; this amount representing the degree of equivalent supersaturation ot' the liquor with respect to these substances at that temperature. Other substances containcd in the liquor were of suliiciently low concentration that the cold liquor was unsaturated therewith.

Large crystals of the hydrated double salt were withdrawn from the bottom of the crystallizer at point 12, Figure 2, at the net rate '5 about (exclusive seed added) lOO kilograms per hour.

The liquor overflowing the crystallizer, carrying in suspension the fine borax crystals, was first passed to a continuous thickening equipment of well-known design. A thickened sludge o-t borax crystals and liquor was then passed to a continuous vacuum filter for the final separation of solids and liquids. Clear liquor from this crystallization and separation process was then returned to evaporator-s for the concentration and recovery of residual values. Essentially pure borax is recovered by these means. By ap plying a displacing wash of water to the borax crystals on the filter for the purpose of removing adhering mother liquor, a high grade borax, free from the phosphate double salt, is obtained, which with the customary drying and packing is suitable for the market.

In certain cases we have found it advantageous to dispense with the thickening equipment, the liquor overflowing the upper part of the crystallizer passing directly to a suitable filter or centrifugal for separation or" the fine crystals from the liquor.

YVhile it is not specifically the purpose of this description to claim details of crystallizer construction, it may be noted that we have found it advisable to provide means for the prevention of channeling of the liquor stream within the crystallizer. Vi hile Figure 2 she s diagrammatically only have found that a plurality or the inlet J. we same circunii'erentiallyspaced apart is an effective aid in securing good liquor distribution. Likewise, a plurality of overflow lines 13 or a circumferential launder ma be en ployed to advantage. Paddles or stirring; mechanism 11 aliixed to a rotating shaft 10,

ployed.

crystal bed mixed and preventing the char.-

neling of liquor through the crystal bed. This stirrer, although operated at a relatively low speed, serves as an aid to the production of the mechanical stimulus desired for the production of the line borax crystals.

i The means provided for preventing short oircuiting or channeling of the liquor also aids in bringing about the desired hydraulic classification Within the crystallizer, not only With-regard to the line boraX crystals, but also with respect to the large seed crystals of the double salt maintained Within the crystallizer. By means of this hydraulic classification, the largest crystals of the seed bed are caused to Work their way to the lower extremity of the crystal izer, from which they may be Withdrawn continually.

The greater portion or" the large crystals of the double salt Withdrawn from the lower extremity of the crystallizer are separated from the relatively small amountof accompanying liquor on a filter, drain table or centrifugal. They may be given a slight Wash with Water for further purification,

then dried and sacked for shipment.

Approximately 40 kilograms per hour of the large seeds are crushed to approximately "5 mesh and returned to the upper portion of the crystallizer for seed. 'lVhile, theoreticall only about four kilograms per hour red, imperfect classification and the removalof crystals or" smaller sizethan 2.31 millimeters requires the addition of this larger quantity of seed.

In the foregoing description of the case wherein the degree of supersaturation With respect to compound beta produced by cooling to Z was in excess 01 the metastable limit, We have shown it to be necessary to reduce this degree of supersaturation in order to insure successful deposition of the compound beta upon the bed of crystals contained with in the body of the crystallizer. in the inst nce LlGSCllbBCl' the solution was cooled only to V for the first crystallizing step, followed by a cooling Z and a second crystallizing step.

lVhile two steps Were selected forillustration, it may be possible that a greater nuirbcr would be required in order to avoid incurring too great a degree of supersaturation in any single step. One disadvantage of such a process resides in the fact that plurality of duplicated units are required. l ve have found, however, that the multistep process may be accomplished incl'lech-in a. single unit. To this end liquor at Z overflowing the crystallizer at point 13 of Figure 2 is in ined to the feed tank 19. To stimuate torinstance, the two-step process hereinefor described, a volume of overflow liquor equal to the volume of fresh liquor is em- Obviously the overflow liquor at Z fort is only saturated with respect to compound beta, containing l) parts per 100 parts of solvent. lliixzng this liquor With an equal volume of fr sh liquor containing B parts of compound beta per 100 parts of solvent, re-

sults in a system containing approximately one hall as much potential supersaturation value as the original liquor or i part: beta per 100 parts of solvent. By these means the desired degree of supersaturation with respect to compound beta may be regulated at Will. The ratio of the overflow liquor circulated to th fresl liquor supplied automatically fixes the degree of supersaturation of the pound beta. Manipulation of the liquo for the purpose of obtaining the desired small tals of compound alpha may be carried out in anyof various methods here'nbefore set forth. By this process of our invention we are able to crystallize at a specified tematuro, and in a single installation of matwo compounds from solution, said ounds having similar solubility chartics, and to separate to a substantial of purity the one compound from the lormation or" fine crystals of the two or more other. compounds While causing the orderly and regulated deposition of the one com- 1; 7 nd upon a predetermined seed bed, according to the principles hereinoefore set The economical limit of manipulao't co so, determined by the value tions of the compounds :i' stake, andby comparison with other chemical and physical separation processcs, made available by the increa number of possibilities.

l rhi'le the particular process herein deseribec is Well adapted to carry out the 0bects of the present invention, it is to be understood that various modifications and changes may be made Without departing l. om the spirit of the invention, and the lllvQUtlOll is of the scope of theappended claims.

We claim:

1. A process of crystallizing simultaneously two different substances from solution which includes, precipitating one substance as relatively line crystals from a solution maintained within the metastable limit of supersaturation with respect to the other substance, While maintaining suficient relatively large crystals of said latter substance prest as to precipi to said latter substance by deposit upon and growth of such relatively teed liquor with respect'to comlarge crystals, whereby the two substances are simultaneously obtained as crystals of distinctly different sizes.

2. A process of separating two substances which crystallize simultaneously from solution which. includes, precipitating one sub stance as relatively line crystals from a solution which is mai taincd w" iin the metastable limit of supersaturation with respect to the other substance, and while maintaining suthcicnt relatively large crystals of said lat-- ter substance in contact with said solution as to precipitate such latter substance by deposit upon and growth ot such relatively large crystals, wh reby the two ditli erent substances are simultaneously ob ained as crystals ot distinctly different sizes, and separating the crystals by hydraulic classification.

3. A process of the described which includes, precijtating one substance from solution as relatively tin-e crystals which solution is maintained within the metastable limit of supersaturation with respect to the other substance, and while passing the solution upwardly through sutlicient relatively large crystals of .7 aid latter substance as to precipitate said latter substance by deposit upon andL growth of such relatively large crystals, the low of solution by such crystals of the latter substance being at a rate sutlicient to carry away the relatively fine crystals of the former substance without carrying away the relatively large crystals of the latter substance.

i. A process of crystallizing substances from solution which comprises, passing the solution of two different substances to be precipitated at one condition of temperature and concentration to a bot y of said liquor at another condi ion of temperature and concentration, such will cause the feed liquor to exceed the metastable limit of supersaturation it brought to such condition of temperature and concentration in one separate operation, while circulatingfrom the body of the liquor in the crystallizing zone suitieient of said liquor to re-enter with the fresh liquor as to maintain the liquor in the crystallizing zone within the metastable limit of supcrsaturation with respect to at least one of the substances to be crystallized, providing relatively large crystals in said crystallizing zone of such substance so as to cause the crystallization of said substance by growth upon said crystals, and precipitating simultaneously from the solution another substance in the term or" relatively tine crystal 5. A process of crystallization which comprises, flowing: a solution by relatively large crystals o't one substance while the solution is in .[llILl within the metastable limit of supersatrra on with respect to said substance, and whi e the solution is precipitating another substance in the form of relatively tine crystals, and supplying fresh solution to the crystallizing zone which said 1h solution contains sufficient of both subes so that it in'nnediately reduced to conditions of temperature and concentration existing in said crystallizing zone, such solution would be labile with respect to both sub stances, and recirculating: sufficient of the solution from the crystallizing zone to reenter with the fresh solution as to prevent the f esh solution from exceeding the metai-ztable l mit of supersaturation with respect to the component corresponding to the relatively large crystals.

6, It process of crystallization which comflowing a solution by relatively large seals of one substance while the solution maintained within the metastable limit supersaturation with respect to said subtle-ice, and while the solution is precipitatither substance in the form of relaine crystals. a d supplying fresh solutim. to the crystallizing; zone which said i h olution contains sufficient of both subo innnediately reduced to conof temp rature and concentration existing: in said erystallizing zone, such solu- 1 would be labile with respect to both x: tances, recirculating suniicient of "he solufrom the crystallizing zone to re-enter the fresh solution as to prevent the 1 solution from exceeding the metastable 1 nit of super aturation with respect to the component corresponding; to the relatively large crystals, and causing a return of flow in the crystallizing zone to be sullicicnt to carry away the relatively tine crystals of one of such substances without carrying away the relatively large crystals of the other substance.

7. A pr cess of crystallizinp; two different substances sin'iultaneously from solution which conu'irises, cooling the solution so as to cause the solution. to become supersaturated with respect to two substances, while m intaining: the solution within the metab e limit of supersaturation with respect to at least one ot said substances. contacting with the solution sutiicient relatively large crystals o't said substances to cause the precipitawin of said substance by growth of the relatively large crystals and causing; the cipitating of the other substance as relaively {inc crystals.

8. A process of crystallizing two different substances simultaneously from solution which comprises, cooling the solution so as to cause the solution to become supersaturated with respect to two substances, while n'iaintaining the solution within the metastable limit of supersaturation with respect to at least one of said substances, contacting; with the solution sutlicieut relatively large crystals of said substances to cause the precipita ion of said substance by growth of the relatively large crystals, causing the precipitating of the other substance as relatively fine crystals, and maintaining a flow of solution by the relatively large crystals suflicient to carry away the relatively small crystals without however, removing the relatively large crystals.

9. A process of crystallizing two different substances simultaneously from solution which comprises, cooling the solution so as to cause the solution to become supersaturated with respect to two substances, While maintaining the solution within the metastable limit of supersaturation with respect to at least one of said substances, contacting with the solution sufficient' relatively large crystals of said substances to cause the precipitating of said substance by growth of the relatively large crystals, causing the precipitation of the other substance as relatively fine crystals, and separating the relatively large and small crystals from each other by hydraulic classification.

10. A process of crystallizing salts from solution which comprises, cooling the solution to a temperature below the saturation point of said, solution with respect to two salts, one of said salts at least being maintained within the metastable limit of supersaturation, flowing said solution by a bed of relatively large crystals of said salt while causing the precipitation of other substance as relatively fine crystals, supplying a "fresh 7 solution to the crystallizing operation, and recirculating av part of the solution in the crystallizing operation back to the relatively large crystals with the fresh solution.

Signed at Trona, California, this 17th day of February, 1930.

7 WILLIAM E. BURKE.

CHARLES E. RITCHIE.

ROBERT B. 'PEET. 

